UCP1-deficiency causes brown fat respiratory chain depletion and sensitizes mitochondria to calcium overload-induced dysfunction

Read Also

UCP1 is a key feature of thermogenic fat cells, both brown and beige. We have demonstrated here that upon cold exposure interscapular BAT of UCP1-KO mice has global mitochondrial disruptions, which extend well beyond the deletion of UCP1 itself. These data reveal physiological interactions between UCP1 and ROS. The role of UCP1 itself in the regulation of ROS production is not fully understood. Evidence in support of a robust role for UCP1-mediated uncoupling in the regulation ROS production in vitro has been provided (6, 7, 41), while findings suggesting a limited role for UCP1 activity in controlling ROS in vitro has also been presented (8, 42-44). Importantly, UCP1 does seem to play a role in regulating BAT redox tone in vivo (9), and acute adrenergic stimulation in vivo drives ROS production to support UCP1-dependent thermogenesis (10).

Our findings demonstrate that UCP1-deficient BAT mitochondria are poorly equipped at buffering calcium in a ROS-dependent manner. Most importantly, we show that the acquired molecular and functional differences between BAT mitochondria from WT and UCP1-KO animals are more widespread than the deletion of UCP1 itself. Considering the striking alterations to the BAT mitochondrial proteome (substantial reduction of ETC abundance) of UCP1-KO mice, caution must be taken when attributing a BAT phenotype solely to UCP1 deletion in these animals. In addition, reduced ETC expression may be commonly associated with decreased UCP1 levels more generally, and should be kept in mind when studying genetic models with reduced BAT UCP1 expression. Notably, our findings here suggest that the reduced capacity of UCP1-KO BAT to activate oxidative metabolism following adrenergic administration (cold or chemical) is at least partly due to reduced expression of the ETC, and not solely due to lack of UCP1-mediated uncoupling.

Examination of calcium sensitivity of BAT mitochondria with and without UCP1 adds further evidence to the relevance of mechanisms of ROS production in BAT. Previous studies have compared ROS production between WT and UCP1-KO BAT mitochondria when using G3P as a respiratory substrate, indicating either comparable (8), or enhanced  (6, 7) levels. Importantly, G3P-mediated mitochondrial energization can drive ROS production by RET or from mitochondrial G3P dehydrogenase (GPD2) itself (7, 30).  Moreover, GPD2 appears to have the capacity to produce ROS in the mitochondrial IMS (30, 41, 45), as opposed to Complex I that produces superoxide in the mitochondrial matrix (12).  This compartmentalization of ROS production is a plausible explanation for the sensitivity of UCP1-KO mitochondria to succinate mediated ROS production, which drives superoxide production principally through complex I (12). Since G3P-mediated ROS production can drive ROS independently of Complex 1 (i.e. at GPD2 itself) (7, 30, 41), our data in sum suggest that Complex I-mediated ROS production by RET is a major contributor to mitochondrial dysfunction in UCP1-KO BAT.  This interpretation is in line with the recognized importance of ROS originating in the mitochondrial matrix supporting permeability transition (46, 47). Interestingly, GPD2 abundance was unaltered in UCP1-KO BAT (Table S1) suggesting that in the absence of UCP1, G3P-mediated electron flux to coenzyme Q (CoQ) is maintained. Previous investigations have noted quantitatively different effects of G3P-driven ROS production in BAT mitochondria between WT and UCP1-KO animals (6-8). In light of our findings, these discrepancies may be predictive of differential mitochondrial adaptation in different UCP1-KO mice colonies to mitigate ROS-sensitivity due to genetic absence of UCP1. Such differences might be expected to arise on congenic (i.e. C57BL/6J and 129/SvImJ) backgrounds, which are particularly sensitive to ablation of UCP1 (48), and may therefore be prone to selection against enhanced ROS production depending on breeding strategy.  More generally, the functional effects that arise from distinct ROS sites in BAT upon thermogenesis is an interesting avenue of research to pursue in the future.

The data presented here indicate that mice genetically lacking UCP1 exhibit a plethora of acquired features that extend substantially beyond the deletion of UCP1 itself. These defects, such as the striking reduction of mitochondrial ETC components should be considered when using this mouse model to study UCP1 function. However, this model may have utility for examination of general features of mitochondrial dysfunction. For example, the molecular processes regulating the discordance between ETC protein and mRNA abundance in cold-exposed UCP1-KO animals may be an appropriate model for studying fundamental mechanisms of mitochondrial proteostasis.